Semipermeable membranes are widely used for the removal of impurities in beverages/industrial water, desalination of seawater, removal of saprophytic bacteria in foodstuffs, and a waste water treatment, or in the field of biochemistry and the like.
For the semipermeable membranes, various polymers such as a cellulose-based resin, a polyvinyl alcohol-based resin, a polysulfone-based resin, a polyamide-based resin, a polyimide-based resin, a polyacrylonitrile-based resin, a polyester-based resin, and a fluororesin are selected in accordance with the use. However, the membrane itself has weak strength, and cannot endure a high pressure such as 1 MPa to 10 MPa or more when used alone in ultrafiltration, reverse osmosis or the like. Thus, products in the form of having a semipermeable membrane formed by applying a resin liquid for a semipermeable membrane on one surface of a support having high strength and high liquid permeability, such as a nonwoven fabric or a woven fabric, are in use.
In order to obtain the liquid permeability and filtration performance required from a semipermeable membrane, it is necessary that a semipermeable membrane be formed at a uniform thickness on a semipermeable membrane support. Therefore, high smoothness is required from the surface where a semipermeable membrane will be coated in the semipermeable membrane support (hereinafter, also referred to as a semipermeable membrane-coated side surface). Furthermore, adhesiveness of the semipermeable membrane to the support (=anchor effect) is also required. However, if the semipermeable membrane support is made excessively smooth, when the semipermeable membrane coating liquid is applied, it becomes difficult for the coating liquid to cling to the support, adhesiveness of the semipermeable membrane to the support becomes poor, and the semipermeable membrane becomes easily detachable from the support. To the contrary, when the smoothness of the support is lowered, it becomes easy for a resin liquid to cling to the support by the anchor effect, and adhesiveness is improved. However, uniformity of the semipermeable membrane is deteriorated, and there occurs a problem that the coating liquid to be applied bleeds into the interior of the support and thereby permeates through to the non-coated surface. That is, in regard to the smoothness of the semipermeable membrane-coated side surface, uniformity of the thickness of the semipermeable membrane and the adhesiveness of the semipermeable membrane to the support are in a contradictory relationship.
It has been suggested to improve the adhesiveness of a semipermeable membrane coating liquid to a support by roughening the coated surface by adjusting the difference in the surface roughness between the semipermeable membrane-coated side surface of a nonwoven fabric for semipermeable membrane support and a non-coated surface to 15% (see, for example, Patent Literature 1).
As a nonwoven fabric for semipermeable membrane support, a support based on a bilayer structure of a front surface layer which uses a fiber having a larger diameter and has large surface roughness; and a back surface layer which uses a fiber having a finer diameter and has a dense structure, has been suggested (see, for example, Patent Literature 2).
As a nonwoven fabric for semipermeable membrane support, there has been suggested a support characterized by containing two or more kinds of main constituent synthetic fibers having different fiber diameters, and being formed from a nonwoven fabric in which a binder synthetic fiber, and the ratio of smoothness between a semipermeable membrane-coated side surface and a non-coated surface is 5.0:1.0 to 1.1:1.0 (see, for example, Patent Literature 3).
There has been suggested a support in which the average value of breaking lengths in the longitudinal direction (MD) and the transverse direction (CD) at the time of 5% elongation is 4.0 km or more, and the degree of air permeability is 0.2 cc/cm2·sec to 10.0 cc/cm2·sec (see, for example, Patent Literature 4).
There has been suggested a support in which adhesiveness to a semipermeable membrane has been increased by incorporating an atypically shaped cross-section fiber on the coated surface side layer of the semipermeable membrane (see, for example, Patent Literature 5).
There has been suggested a support having a three-layer structure in which an intermediate layer includes a melt-blown fiber having a fiber diameter of 5 μm or less (see, for example, Patent Literature 6).
There has been suggested a support in which prevention of the permeation-through of a semipermeable membrane coating liquid is attempted by incorporating pulp for papermaking into a layer on the non-coated surface side of the support having a multilayer structure (see, for example, Patent Literature 7).